In a two transmitter prior art radar apparatus example, first and second transmit signals are known by a control device, which can apply control signals to change the characteristics of, in one example, two receivers. The first and second transmit signals, or characteristics thereof, may be stored in a computer memory or memories. The control device of this example prior art radar apparatus time reverses the known first and second transmit signals and thereby generates corresponding first and second matched filter signals, which are used as first and second control signals, by the first and second receivers, respectively.
In this prior art radar apparatus, the first matched filter signal, which is based on the known first transmit signal but not on the known second transmit signal, is used as the first control signal to determine an impulse response or characteristic of a first receiver. The second matched filter signal, which is based on the known second transmit signal but not on the known first transmit signal, is used as a control signal to determine an impulse response or characteristic of a second receiver. In this example prior art apparatus, the first control signal (also called the first matched filter signal) separates out a first transmit signal component from a first return signal, received through the airwaves, received at an input of the first receiver. Similarly, the second control signal (also called the second matched filter signal) separates out a second transmit signal component from a second return signal, received through the airwaves, received at an input of the second receiver.
In the this simplified prior art example, the first return signal received through the airwaves, at the input of the first receiver, is filtered or acted on by the first receiver (as controlled by the first control signal) and the first receiver thereby modifies the first return signal received at its input to produce an output signal at the output of the first receiver. Similarly, the second return signal received through the airwaves, at the input of the second receiver, is filtered or acted on by the second receiver (as controlled by the second control signal) and the second receiver thereby modifies the second return signal received at its input to produce an output signal at the output of the second receiver.
In this prior art example, this is not the end of the process. Rather a typically multiple iterative process is executed in order to find appropriate first and second transmit signals. Based on an iterative back-projection algorithm between the time and frequency domain, the first transmit signal, to be transmitted through the airwaves by the first transmitter is iteratively modified to a different transmit signal, and consequently the known first transmit signal at the control device will also change. This process will be repeated until the output signal at the output of the receiver satisfies some criteria with respect to the stored known first transmit signal.
Similarly, the second transmit signal, to be transmitted through the airwaves by the second transmitter is changed to a different transmit signal, and consequently the known second transmit signal at the control device will also change. This process will be repeated until the output signal at the output of the receiver satisfies some criteria with respect to the stored known second transmit signal.
This iterative process of the example prior art radar apparatus is inefficient and has what other disadvantages. Various transmit signals, for transmitting through the airwaves, have been used or have been suggested to be used in radar systems. For example, in a series of papers, Stoica et. al. has suggested using unimodular sequences with good auto/cross correlation properties, for use as transmit signals in radar systems [P. Stoica, J. Li and X. Zhu, “Waveform Synthesis for Diversity-Based Transmit Beampattern Design,” IEEE Transactions on Signal Processing, Val. 56, Issue 6, June 2008; P. Stoica, H. He, and J. Li, “New Algorithms for Designing Unimodular Sequences With Good Correlation Properties,” IEEE Transactions on Signal Processing, Val. 57, No. 4, April 2009; H. He, P. Stoica, J. Li, “Unimodular Sequence Sets with Good Correlations for MIMO Radar”, 2009 IEEE Radar Conference, Pasadena, Calif. USA, May 4-8, 2009